"This is inside us!"
One of the options said: " A woman inside us!"
This whole lesson is a riot! Hahaha!
I think this phrase is more like "there is among us". "Inside us" would be בפנינו, wouldn't it?
The word בתוך doesn't mean among, it means inside (....of something).
The word בפנים is "inside" on its own, and doesn't get pronoun endings attached to it.
So for example:
אני בפנים. אני הולך החוצה.
I am inside. I am going outside.
אני אוהב את המרק הזה. יש בתוכו פלפלים.
I love this soup. There are peppers inside it. (Or more naturally "it's got peppers in it").